But the Home Office said the bill would just maintain existing powers relating to postal data.

And it stressed that only data about mail - not its contents - would be retained if the law was ever enacted.

Under the draft bill, the Royal Mail and other postal services could be asked to retain "anything written on the outside" of items for up to 12 months so they can be accessed by the police, security services and HM Revenue and Customs.

But a Home Office spokesman said only information relating to the "communications data of mail — who sent the letter to whom, when it was sent and the origin and destination" would, potentially, be stored.

'Snooper's charter'

Asked how the relevant data would be separated from the contents of postcards, for example, the spokesman said: "That would be something the postal services would need to address if they were requested to do so."

The spokesman added: "It does not cover the interception of contents, which will continue to be covered by the Regulation of Investigatory Powers Act and require a warrant signed by the Secretary of State.

"In a very small number of cases, law enforcement or intelligence agencies might have to obtain the communications data of a suspect's mail, but there are currently no requirements to retain postal data and there are no plans for that to change.

"The new legislation will replace current powers relating to postal communications data. The Bill will ensure they are maintained."

Detail of all UK internet use will be stored for up to 12 months so that it can be accessed by the police, the security services and HM Revenue and Customs if the draft bill becomes law.

Records will include people's activity on social network sites, webmail, internet phone calls and online gaming.

It has been dubbed a "snooper's charter" by civil liberties groups but the Home Office says new powers are needed to keep pace with how criminals and terrorists are using new technology.

The Bill includes provision to help postal services and other communications providers with the cost of installing new equipment to comply with any laws, estimated to be £1.8bn over 10 years.